1. Field of the Invention
The present invention relates to a material for ocular lenses and a process for producing the same. The present invention also relates to ocular lenses comprising the above ocular lens material and copolymers suitable as the ocular lens material. The ocular lens materials, as well as the ocular lenses, according to the present invention cause no or very little elution therefrom, and have a high water-retaining capability, oxygen permeability and excellent transparency, as well as resistance to deposition, mechanical strength and flexibility. As a result, the ocular lens materials and ocular lenses of the present invention correct visual acuity and have excellent durability, safety, comfort during wear and handleability, and are suitable as soft contact lens, intraocular lens and artificial crystal lens.
2. Description of the Background
Copolymer hydrogels principally comprising 2-hydroxyethyl methacrylate have been widely used for producing soft contact lenses. This is because 2-hydroxyethyl methacrylate copolymers, having excellent machine-cuttability and polishability, are easily processed into lenses, and the hydrogels obtained by hydration and swelling the copolymers have appropriate strength and flexibility.
However, hydrogels of 2-hydroxyethyl methacrylate copolymers have, in general, a low water content of less than 40% by weight and soft contact lenses formed therefrom have poor oxygen permeability. Soft contact lenses from the 2-hydroxyethyl methacrylate copolymers therefore fail to allow oxygen to be supplied to the cornea in a physiologically sufficient amount. Consequently, these soft contact lenses may, when worn for a long period of time, cause the cornea tissue to suffer from anoxia, which eventually leads to eye injury.
Attempts have therefore been made to develop a soft contact lens having high oxygen permeability and a material therefor, which can replace the 2-hydroxyethyl methacrylate copolymer hydrogels. Representative examples so far developed are as follows:
(1) A process for producing polymeric materials having a dually crosslinked structure usable for ocular lens. On producing a crosslinked polymer from two types of vinyl monomers which are hardly copolymerizable with each other, such as a combination of an N-vinyllactam and a (meth)acrylate, two types of crosslinking agents are used, one having a high copolymerizability with one of the two monomers, and the other with the counterpart monomer (Japanese Patent Application Laid-open Nos. 3487/1975 and 140594/1975).
(2) A soft contact lens with high water content obtained by copolymerizing a composition principally comprising methylacrylamide and a fluoroalkyl (meth)acrylate and then shaping the resulting copolymer (Japanese Patent Application Laid-open No. 293520/1988).
(3) A copolymer obtained by copolymerizing a polymerizable composition containing specific amounts of N,N-dimethylacrylamide and a specific fluoroalkyl (meth)acrylate and, as necessary, not more than a specific amount each of another copolymerizable vinyl monomer and a crosslinking agent; and a contact lens comprising the copolymer (Japanese Patent Application Laid-open No. 70713/1990, U.S. Pat. Nos. 4,954,587 and 5,011,275).
However, the polymeric material obtained by (1) has no resistance to deposition. This is essential for ocular lenses such as contact lens, and when a soft contact lens comprising the material is worn, significant deposition occurs. As a result, the lens may cause cornea injury or may not correct visual acuity sufficiently.
Furthermore, the contact lenses obtained by (2) and (3) have very low strength and readily break (no durability), and cannot be used in practice.
Contact lenses, which directly touch the eyes, are required to generate no eluates, in order to prevent eye injury and provide sufficient ability to correct visual acuity. The polymeric material obtained by (1), using the two types of crosslinking agents, can suppress the amount of eluate therefrom to some extent, but insufficiently.
There has been proposed, in order to provide a low-elution material for contact lenses:
(4) A process for producing hydrophilic polymers for soft contact lenses, which comprises polymerizing a methacrylic acid ester and an N-vinyllactam in the presence of a crosslinking agent, such as vinyl methacrylate, vinyl acrylate, triallyl isocyanulate or dicarboxylic acid divinyl esters (Japanese Patent Application Laid-open Nos. 105992/1977 and 74049/1978, U.S. Pat. No. 4,184,992). However, process (4) sometimes leads to formation of an opaque polymer, which is not satisfactory for ocular lenses requiring high transparency.
There has been strong demand for an ocular lens material suitable for soft contact lenses, having high water retaining capability and oxygen permeability, as well as excellent transparency, resistance to deposition, mechanical strength and flexibility, and which causes no elution. As a result of an intensive study to develop an ocular lens material having the above features, the present inventors had found the following materials and applied patents therefor (Japanese Patent Application Laid-open Nos. 134029/1994, 53639/1995, 239431/1996 and 278832/1997).
These materials include: (i) a hydrogel comprising a copolymer obtained by polymerizing a polymerizable hydrophilic monomer, N-hydroxymethylacrylamide and a crosslinking agent; (ii) a water-retaining ocular lens material comprising a copolymer of an N-vinyllactam and a specific fluoroalkyl (meth)acrylate; (iii) a water-retaining ocular lens material comprising a copolymer obtained by polymerizing a monomer mixture containing N-acryloylmorpholine, a fluorine-containing unsaturated monomer and a crosslinking agent; and (iv) an ocular lens material comprising a copolymer obtained by polymerizing a monomer mixture containing an N-vinyllactam, N,N-dimethylacrylamide, a fluoroalkyl (meth)acrylate and a crosslinking agent. These materials all have high water retaining capability and oxygen permeability, as well as excellent transparency, resistance to deposition, strength and flexibility and are hence very suitable as materials for ocular lenses, such as soft contact lens.